Film thickness sensor with porous blower

Abstract

A sensor system for measuring the thickness of flat material that is moved relative to the sensor system has a first sensor device for measuring the thickness of the flat material and a device for generating an air cushion. The device is disposed in such a way that there is an air cushion between at least one side of the sensor device that faces the flat material, and the flat material, during operation. In the region of the air cushion, the first sensor device includes surface sections having porous material and/or material that is provided with micro-holes.

What is claimed is: 
     
       1. A sensor system for measuring a thickness of a flat material that is moved relative to the sensor system, said sensor system comprising:
 a first sensor device for measuring the thickness of the flat material, which is configured as a blown film bubble during the measuring, from outside the film bubble; and 
 a device for generating an air cushion between the flat material and at least one side of the first sensor device that faces the flat material during the measuring, 
 the first sensor device including (i) in a region of the air cushion, surface sections that include at least one of a porous material and a material that is provided with micro-holes, and (ii) a capacitive measuring device having at least two electrodes disposed on the at least one side that faces the flat material, 
 the air cushion being located inside a nominal radius R of the film bubble, and 
 the surface sections of the first sensor device that include at least one of the porous material and the material that is provided with micro-holes having holes with a hole size of less than 100 μm. 
 
     
     
       2. The sensor system according to  claim 1 , wherein the first sensor device includes inductive measuring devices. 
     
     
       3. The sensor system according to  claim 1 , wherein the at least one of the porous material and the material that is provided with micro-holes is disposed in at least one section of the at least one surface of the first sensor device that faces the flat material, selected from
 in a region between active surfaces of the electrodes that face the flat material, 
 around the active surfaces of the electrodes that face the flat material, and 
 on the active surfaces of the electrodes that face the flat material. 
 
     
     
       4. A blown film line comprising a sensor system according to  claim 1 , the sensor system being configured to measure the film thickness at at least one location of the blown film line selected from
 between a blown film die and a frost region of the film tube, 
 at a calibration basket, 
 between the calibration basket and a flatness unit, 
 at the flatness unit, 
 at a squeezing device, and 
 after the flatness unit. 
 
     
     
       5. The sensor system according to  claim 1 , wherein a penetration depth of the air cushion into the nominal radius R of the film bubble is from 1 to 15 centimeters. 
     
     
       6. The sensor system according to  claim 1 , wherein the hole size is less than 80 μm. 
     
     
       7. The sensor system according to  claim 6 , wherein the hole size is less than 50 μm. 
     
     
       8. The sensor system according to  claim 1 , further comprising a second sensor device for detecting damage or holes in the flat material, the second sensor device being disposed relative to the first sensor device and the flat material so as to detect the damage or holes in the flat material before the damage or holes reach the region of the air cushion due to relative movement between the flat material and the first sensor device. 
     
     
       9. The sensor system according  claim 8 , further comprising a return motion device for changing a distance between the first sensor device and the flat material. 
     
     
       10. A method of operating a blown film line for producing a film, comprising:
 measuring a thickness of the film, which is configured as a blown film bubble during the measuring, from outside the film bubble with a first sensor device during operation of the blown film line, and 
 generating an air cushion between the film and at least one side of the first sensor device that faces the film, 
 the first sensor device including (i) in a region of the air cushion, surface sections that include a porous material or a material that is provided with micro-holes, and (ii) a capacitive measuring device having at least two electrodes disposed on the at least one side of the first sensor device that faces the film, with the surface sections of the first sensor device that include at least one of the porous material and the material that is provided with micro-holes having holes with a hole size of less than 100 μm, and 
 the air cushion (i) being generated by conveying air through the porous material or the material that is provided with micro-holes and (ii) being located inside a nominal radius R of the film bubble. 
 
     
     
       11. The method according to  claim 10 , wherein the side of the first sensor device that faces the film is positioned by a holding device against the film bubble during a measuring period such that the surface of the first sensor device that faces the film is positioned inside the nominal radius R of the film bubble. 
     
     
       12. A sensor system for measuring a thickness of a flat material that is moved relative thereto, comprising:
 a first sensor device for measuring the thickness of the flat material; 
 a device for generating an air cushion between the flat material and at least one side of the first sensor device that faces the flat material, 
 the first sensor device including in a region of the air cushion, surface sections that include at least one of a porous material and a material that is provided with micro-holes, with the surface sections of the first sensor device having holes with a hole size of less than 100 μm; and 
 a second sensor device for detecting damage or holes in the flat material, the second sensor device being disposed relative to the first sensor device and the flat material so as to detect the damage or holes in the flat material before the damage or holes reach the region of the air cushion due to relative movement between the flat material and the first sensor device. 
 
     
     
       13. The sensor system according to  claim 12 , further comprising a return motion device for changing a distance between the first sensor device and the flat material.